Since the integration degree of semiconductor devices has increased, the size of semiconductor devices has been reduced. Accordingly, the size of contact holes connecting conduction wirings has been also reduced. One of the methods for overcoming the limitations of the photolithography process for forming contact holes is a method of forming a self-aligned contact hole using, for example, a nitride layer as an etching barrier layer.
In general, an interlayer insulating layer of a semiconductor device is formed of an oxide layer. As the aspect ratio of self-aligned contact holes is increased and the pitch thereof is reduced, a void can be generated when gap-filling the interlayer insulating layer. In order to gap-fill the interlayer insulating layer without any voids, an oxide layer doped with a dopant is used. As the pitch of contact holes is reduced, the amount of the dopant doped in the oxide layer is also gradually increased.
After forming self-aligned contact holes by etching the oxide layer highly doped with a dopant, a cleaning process is performed to remove by-products created in the etching. During the cleaning process, the loss of the insulating layer doped with the dopant is increased, thereby reducing the isolation margin between the contacts holes.